How use SetIntArrayRegion with std::vector int ?

Question

I have a vector of ints that i want to put into a jintArray. How do I do this?
When I tried the below code, it didn't work but instead when i printed out the int values on the java side, they were all messed up.
jintArray arr = env->NewintArray( intVector.size() );
env->SetintArrayRegion( arr, 0, intVector.size(), ( jint * ) &( intVector ) );This returns weird values. What am I doing wrong?

Answer 1

This isn't a Java/JNI-specific issue. Your code also wouldn't work if you tried to do this:
void foo( int* );
void bar() {
    vector<int> a(10);
    foo( (int*)&a );
}Hint: using a cast is a bad idea. It's wrong for the same reason the following code is wrong:
void foo( double* );
void bar() {
    double d = 42.0;
    foo( (int*)&d );
}You can't just use a cast as a sledge-hammer and magically expect the right thing to happen. In your original code, you're passing the address of the C++ vector object as if it were the address of a plain old C-style array of int.
Using old-style casts is a bad idea in general. This is why new-style casts were added to the language. Had you tried to use:
    static_cast<int*>( &intVector )the compiler would have correctly complained. Old-style casts assume you know what you're doing. In this case, your old-style cast is equivalent to a reinterpret_cast<int*>.
That aside, there's intentionally no way to convert a vector<T> into the address of its first element because there's no guarantee that the implementation of vector will store array elements contiguously.